Polyurethane coatings are known in the industry as some of the toughest and most useful coatings available. Polyurethane coatings are versatile and have wide-ranging applications as commercial and industrial protective coatings as well as for use as decorative coatings on walls, floors and other surfaces.
Moisture-curing polyurethane coatings are a highly desirable group of such polyurethane coatings. Moisture-curing polyurethane coatings advantageously can be produced without the use of heat or other external sources of energy being applied. Such coatings are very useful, for example, as paints provided to coat buildings and equipment that cannot be heated. Typical applications for such paints could include painting of prefabricated metal buildings, machinery, aircraft, vehicles, ships, etc. The tough film formed by the cured polyurethane coating forms a barrier resisting corrosion and damage to the coated surface. Such moisture-curing polyurethane coatings comprise extended polyols combined with polyisocyanates that react with atmospheric water at room temperatures to cure and form useful films. These coating compositions may be pigmented as desired and additives (such as catalysts) may be incorporated to control composition properties, such as the time required for curing of the coating. Moisture-curing polyurethane coatings are typically available in single component and two component systems. Examples of such moisture-curing polyurethane coatings are taught in my U.S. Pat. Nos. 4,304,706 (Urs) and 5,998,532 (Urs) each of which is incorporated herein by reference.
In certain applications it is essential to closely control the reflectance of light from the surface of the cured film formed by the moisture-curing polyurethane coating. One such application involves the use of moisture-curing polyurethane coatings as paints for use on civilian and military vehicles and equipment. Such vehicles and equipment are often painted in muted colors so as to blend in with the surroundings in which the vehicles and equipment are intended for use. For example, paints with earth-tone colors are often applied to automobiles, utility vehicles and trucks to enable those vehicles to better blend in with the vegetation and terrain in which the vehicle is to be used. By way of further example, paints with grey and blue color tones are often used to coat the hull and exterior surfaces of military water craft to permit such vessels to more closely resemble the water and horizon. Moisture-curing polyurethane coatings may also be formulated specifically as camouflage paints provided for the express purpose of making the vehicles and equipment more difficult to detect under visual and infra-red conditions.
In these and other applications, polyurethane coatings are required to have very low light reflectance properties such that the coating scatters essentially all incident light. For example, the United States military has implemented military specifications MIL-C-53039 and MIL-C-46168 which, respectively, set requirements for light reflectance and other physical properties for two component and single component moisture-curing polyurethane coating systems. Both of the aforementioned specifications require that the film formed by the cured coating have a gloss value of less than 1 measured using a glossmeter at an angle of 60° to the coated surface. The specifications further require that such film have a sheen value of less than 3.5 measured using a glossmeter at an angle of 85° to the coated surface. As is known to persons of skill in the art, these gloss and sheen properties define a coating with a pronounced matte finish and an appearance which is flat with highly limited light reflectance.
Matting agents are typically added to moisture-curing polyurethane coatings in order to obtain the desired low levels of gloss and sheen. However, selection of a matting agent must be made carefully because the matting agent has a material effect on both the cost of the coating as well as on the physical properties of the coating. Typical matting agents may include, by way of example, polymeric organic particles or crystalline silica. Polymeric organic particles are disadvantageous, however, because the particles are unduly expensive and can vary in physical properties from lot to lot making it difficult to formulate consistent batches of paints or coatings. Crystalline silica is a matting agent manufactured by calcining deposits of diatoms to produce a very fine powder which is effective at scattering light when used in a coating composition. Crystalline silica is disadvantageous, however, because special equipment is required to avoid contact between people involved in the manufacturing process and the crystalline silica powder. It is expected that crystalline silica will be phased out for use as a matting agent in future military coating specifications due to these use-related inefficiencies associated with the material. A substitute for crystalline silica is required.
Polyurethane coatings including other conventional flattening or matting agents, such as talc, mica, calcium carbonate, aluminum silicate and non-crystalline synthetic silica, have demonstrated reduced levels of gloss and sheen. However, the quantities of matting agents required to reduce the gloss and sheen to within the abovementioned military coating specifications are large enough so as not to be practical. The amount of matting agent incorporated into a polyurethane coating must be selected so that the agent will not exceed the critical pigment volume concentration (CPVC). Exceeding the CPVC is disadvantageous because the resultant composition will have decreased flow properties while the cured film will undesirably have decreased film flexibility (impairing formulation and application of the coating) and increased film porosity rendering the cured film more brittle and susceptible to degradation.
A further issue confronting formulators of moisture-curing polyurethane coatings is the need to provide additives which reduce the time and expense of manufacturing the coating compositions. Such additives may be used, for example, to accelerate the various reactions or reduce the temperature conditions required for such reactions to occur. Metal-based catalysts commonly used in the industry are useful in imparting these properties to the coating compositions. However, such catalysts have side effects which are detrimental to the properties of the finished coating composition. For example, use of metal-based catalysts to accelerate the reaction of the polyurethane coating and atmospheric moisture reaction is known to increase the gloss of the finished form film.
There remains a need for a pigmented moisture-curing polyurethane coating which will include all of the performance benefits of existing coatings yet provide a film, following curing, with highly controlled light reflectance properties and which may include additives permitting controlled manufacture and application of the coating compositions in ways which are easy, efficient and inexpensive.